1) Field of the Invention
The invention relates to novel melamine-formaldehyde condensates having thermoplastic properties, a process for the preparation thereof, the use thereof and a method for the analysis thereof.
2) Description of the Related Art
Melamine-formaldehyde resins are used mainly in the wood industry as binders in plywood boards, in laminates, as crosslinking agents in finishes and as bonding material for compression molding compounds. These conventional resins are highly crosslinked and therefore rigid and hard thermosetting plastic materials. The crosslinking takes place as a rule even at low temperatures, so that the materials cannot be melted without crosslinking and thermoplastic processability is therefore not possible.
In the past, various approaches were chosen for improving the thermoplastic processability of aminoplasts, such as melamine-formaldehyde resins.
One approach consists in the transetherification of already etherified melamine resins (WO 03/106558). There, first melamine-aldehyde precondensates are etherified with low molecular weight alcohols and then transetherified with higher molecular weight alcohols. The crosslinked resins prepared in this manner have good melting behavior, i.e. there is a sufficiently large processing window between their melting point and the temperature at which curing begins.
A disadvantage in the case of the known etherified melamine resins is that they are strongly crosslinked and therefore have a relatively high viscosity even in the melt. On incorporation of fillers, the viscosity increases further so that a homogeneous distribution of the fillers in the resin is scarcely possible. Furthermore, the high viscosity results in poor flow behavior of the resin melt in the processing mold.
Another approach for increasing the flexibility of melamine resin molding materials is described in WO 2005/010097. Here, a crosslinked melamine resin ether is mixed with a crosslinked thermoplastic. Crosslinked thermoplastics, such as an ethylene-vinyl acetate copolymer or an ethylene-acrylate copolymer, act as lubricants and thus improve the flow behavior of the molding materials. A disadvantage is that the mechanical properties of the molding materials deteriorate as a result of the addition of thermoplastics.
A further approach for changing the properties of melamine resins consists, for example, in carrying out the condensation reaction with already modified melamine derivatives. Thus, the use of N-alkylated melamine derivatives for the preparation of the polymer products and the condensates thereof with formaldehyde is already known.
The US applications 2005/0282994 and 2005/0014869 describe N-alkylmelamine-formaldehyde compositions which are used as crosslinking agents in finish systems. In particular di- and trialkylmelamines or mixtures thereof are described and the crosslinking thereof with carbamates or compounds which contain active hydrogen.
U.S. Pat. Nos. 3,056,755 and 2,980,637 describe the use of di- and trialkylmelamines and the condensates thereof with formaldehyde and in some cases also alcohols as a constituent of baking enamels. Here, however, the resins described are always only one constituent, which is generally used for crosslinking of alkyd resins. The uses of these condensates are limited to finish systems in which the melamine products are used as crosslinking agents. The condensates are suitable for the formation of crosslinked polymers which as a rule are not processable or only very poorly processable in the melt. Furthermore, in none of these cases are thermoplastic properties of the melamine-formaldehyde condensates reported.